Corrosion causes substantial damage to devices, vehicles and infrastructure. In many cases, protective coatings are available to prevent corrosive environments from oxidizing exposed metals. However, in other cases, such coatings are unavailable or difficult to apply. This is particularly true with portable consumer electronic devices, such as cell phones, which have significant circuitry exposed to the environment, with modest voltages imposed over short distances. If the circuits in these device become immersed, especially in salt water, but even in tap water or rain, corrosion can occur which degrades performance or makes the device effectively unusable. In some cases, the faults that result are intermittent or difficult to diagnose.
Metals and alloys are commonly used as construction materials, in building, bridge, ship, oil, automotive, electric and microelectronic industries. Among others, magnetic metals, such as iron, nickel, cobalt and their alloys, are extensively used in transformers, permanent magnets and magnetic recording media. Corrosion of these media is an important problem affecting daily lives with economical consequences. There are needs for continuous improvement in the methods of corrosive detection and corrosion monitoring.
Corrosion in the atmosphere depends on air moisture and airborne impurities. To reduce the corrosion rate it is critical that corrosive species and dirt are not allowed to accumulate on the surface and thus create a wet poultice. Otherwise, this creates a situation similar to a crevice with possibility for decreased pH and high concentration of corrosive species like chlorides in a marine atmosphere, where corrosion can proceed uninterrupted. The atmosphere environment normally is classified into three categories; rural, industrial and marine. The corrosivity can vary a lot within each class depending on climate, level of pollution, distance to the sea, etc. In addition some atmospheres may be combinations of marine and industrial. In the oil and gas industry corrosion processes involve mainly water and H2S. The corrosivity of fresh waters depends on the hardness and pH of the water, chloride and heavy metal concentration in addition to temperature, flow rate and O2 concentration. In sea waters the main factor is the high content of chlorides. In aqueous environments microbiological organisms play an important role in corrosion. Soil corrosion, which is ascribed to low pH, stray currents, reactive chemicals, low resistivity and bacterial action, may be considered to encompass all corrosion taking place on buried structures.
Traditionally, corrosion in materials as in oil pipeline, offshore structures and vessels is monitored by various techniques such as ultrasonic thickness gauges, Eddy current imaging, X-ray tomography, and electrochemical spectroscopy techniques, designed to analyze interior microstructure and subsurface features. These techniques are less sensitive and some are quite expensive and inconvenient due to sophisticated equipment involved. There are also techniques based on in situ monitoring such as piezoelectric oscillator to measure changes in materials properties. Since the typical sensor sizes are about several micrometers, by distributing these sensors throughout a corroding surface one will be able to examine, with micrometer resolution, if the corrosion occurs locally (at micron scales) or in an uniform fashion.
Out of all the problems associated with corrosion, three aspects of corrosion detection in structures are important: (1) first, one should be able to sensitively detect the extent of corrosion taking place in a given structure at a given location and time, (2) second, one should be able to assess the total damage that has occurred and (3) third, one should assess remnant useful performance left in the structural component of the structure of interest.
The relative constancy of normal corrosion rates of different materials shows that most of the parameters for time of wetness and enhanced corrosion conditions are well defined if the composition of the corroding liquid and the involved metal is fixed. To attain more accurate data of atmospheric corrosion conditions parameters such as relativity humidity, pH and composition of condensing media and temperature need to be monitored. The development of atmospheric corrosion monitors was started a few decades ago in US and in Sweden and are usually called “time of wetness sensors” (TOWS)[1]. These designs are based on galvanic current measurement and they simply determine the periods of time during exposure of the sensors when condensation of moisture occurs on the sensors (>87% RH or relative humidity level) and leads to passage of current in a circuit, the condensate being used as a part of the circuit. In most recently manufactured corrosion sensing units, using bi-metallic thin film micro-sensors performs the monitoring of corrosion in an aggressive environment. The sensor output is essentially a polarized corrosion current of the active component in the couple (working electrode) and does only a qualitative measurement. Now such type of sensors is being employed as atmospheric corrosion sensors (ACS). It is also possible to do quantitative potentiometic measurement and corrosion rate determination by electrochemical linear polarization technique as a function of time and environmental conditions. Generally one potential close to the corrosion potential of the metal is chosen by the others and from the current passing through at this offset potential the corrosion current is determined by employing the Stern-Geary equation[1]. However, in all these methods no consideration was given to the corrosion product buildup and the corrosion potential variation as a function of time and the corrosion process itself. As a result the inferences obtained and the data generated could not be construed as very accurate.
ASTM G 31 7 is an immersion test procedure that typically requires extended exposure periods, up to years, to evaluate the resistance of materials, which are employed in the implant device industry, to corrosion. As such, accelerating the testing by inducing changes on the material and monitoring the results has been an industry-accepted method. The development of accelerated electrochemical tests to study corrosion began in earnest during the late 1960's and into the 1980's in the CPI. The original test standard, ASTM G 5, 8 was expanded from a stepped potentiostatic test to a potentiodynamic test as electronics developed, and subsequently to a cyclic experiment to examine susceptibility to localized pitting and crevice corrosion [ASTM G 61 9]. Galvanic studies between mixed metals in contact with each other were evaluated using the established ASTM G 71 10. See, Richard A. Corbett, “Laboratory Corrosion Testing of Medical Implants”, Corrosion Testing Laboratories, www.corrosionlab.com/papers/medical-implant/medical-implant-testing.htm.
Corrosion sensors are typically electrochemical, and rely on changes in conductivity, resistivity, or redox potential to determine or predict corrosion. See, G. D. Davis, C. M. Dacres, and M. B. Shook, “Development of an electrochemistry-based corrosion sensor to monitor corrosion of boiler tubes, pipes, and painted structures”, DACCO SCI, INC. (www.daccosci.com/SPIE98.htm), expressly incorporated herein by reference.
Corrosion monitoring is the practice of measuring the corrosivity of process stream conditions by the use of “probes” which are inserted into the process stream or environment to be measured and which are continuously exposed to the conditions. Corrosion monitoring “probes” can be mechanical, electrical, or electrochemical devices. These “probes” serve as a proxy for other elements of the system which are presumed to corrode in a corresponding manner to the probes. In some cases, the probe may be an actual element of interest, and indeed, all potentially corroding elements may be sensors. See, www.corrosionsource.com/technicallibrary/condoctors/Modules/MonitorBasics/Types.htm. The nature of the sensors depends on the various individual techniques used for monitoring but often a corrosion sensor can be viewed as an instrumented coupon. In older systems, electronic sensor leads were usually employed for these purposes and to relay the sensor signals to a signal-processing unit. Advances in microelectronics are facilitating sensor signal conditioning and processing by microchips, which can essentially be considered to be integral to the sensor units. Some corrosion measurement techniques can be used on-line, constantly exposed to the process stream, while others provide off-line measurement, such as that determined in a laboratory analysis. Some techniques give a direct measure of metal loss or corrosion rate, while others are used to infer that a corrosive environment may exist. Real-time corrosion measurements refer to highly sensitive measurements, with a signal response taking place essentially instantaneously as the corrosion occurs. Numerous real-time corrosion monitoring programs in diverse branches of industry have revealed that the severity of corrosion damage is rarely uniform with time. Complementary data from other relevant sources, such as process parameter logging and inspection reports can be acquired together with the data from corrosion sensors, for use as input to the management information system.
Known techniques include:
A. Direct techniques
1) Corrosion Coupons (intrusive) (samples are monitored for weight loss due to corrosive loss of material).
2) Electrical Resistance (ER) (intrusive) (measures the change in electrical resistance of a metallic element immersed in a product media relative to a reference element sealed within the probe body).
3) Inductive Resistance Probes (intrusive) (Mass changes in a sensor element are detected by measuring changes in the inductive resistance of a coil, located inside the element).
4) Linear Polarization Resistance (LPR) (intrusive) (a small voltage, or polarization potential, is applied to an electrode in solution and the current needed to maintain a specific voltage shift (typically 10 mV) is directly related to the corrosion on the surface of the electrode in the solution. By measuring the current, a corrosion rate can be derived.)5) Zero Resistance Asymmetry (ZRA) (intrusive) (In ZRA, also known as galvanic monitoring, two electrodes of dissimilar metals are exposed to the process fluid. When immersed in solution, a natural voltage (potential) difference exits between the electrodes. The current generated due to this potential difference relates to the rate of corrosion which is occurring on the more active of the electrode couple.)6) Electrochemical Impedance Spectroscopy (EIS) (intrusive) (an alternating potential perturbation is applied to one sensor element in a three-element probe, with a resultant current response, to measure the impedance.)7) Harmonic Analysis (intrusive) (Higher order harmonic analysis of EIS method)8) Electrochemical Noise (EN) (intrusive) (EN corrosion monitoring tracks extremely small current and voltage fluctuations among three electrodes, made of material as similar as possible to the waste tank material, placed in the waste solution. Current is measured between two electrically coupled electrodes (a working electrode and a counter electrode), while the third electrode is connected between the working electrode and a pseudo-reference electrode to measure the voltage. The magnitude and polarity of the signals, as well as the relationship of the timed signal traces to each other, provide indicators of type and significance of the corrosion processes occurring in the tank. Particular types of corrosion have unique and potential “signatures” that indicate when pitting or stress corrosion cracking is occurring.)9) Potentiodynamic Polarization (intrusive) (measurement of current through a sample with a change in applied voltage potential. An initial static potential is measured, and a scan made from an initial potential of 100 mV below the one-hour potential, in the positive (active to noble) direction at a rate of e.g., 0.16 mV per second, and then reversed when the current has reached two decades greater than that of the breakdown potential [Vb] (defined as a rapid increase of current per increment of applied potential). The reverse scan is stopped when the current becomes less than the current in the forward scan (defined as the protection potential, Vp) or the potential reaches the initial potential. The data is plotted on an x-y semi logarithmic diagram with the current density on the x-axis [logarithmic] in mA/cm2 and the potential versus a SCE on the linear y-axis. Appropriate references in their final form and finish are used as controls.)10) Thin Layer Activation (TLA) and Gamma Radiography (intrusive or non-intrusive) (In this technique developed from the field of nuclear science, a small section of material is exposed to a high energy beam of charged particles, to produce a radioactive surface layer. For example, a proton beam may be used to produce the radioactive isotope Co-56 within a steel surface. This isotope decays to Fe-56, with the emission of gamma radiation. The concentration of radioactive species is sufficiently low, that metallurgical properties of the monitored component are essentially unchanged. The radioactive effects utilized are at very low levels and should not be compared to those of conventional radiography. The change in gamma radiation emitted from the surface layer is measured with a separate detector to study the rate of material removed from the surface. The radioactive surfaces can be produced directly on components (non-intrusive) or on separate sensors).11) Electrical Field Signature Method (EFSM) (non-intrusive) (The technique measures corrosion damage over several meters of an actual structure, clearly distinguishing it from other smaller sensor systems. An induced current is fed into the monitored section of interest and the resulting voltage distribution is measured to detect corrosion damage. An array of pins is attached strategically over the structure for measuring purposes. Increased pin spacing implies lower resolution for localized corrosion. Typical applications involve pin attachment to the external surface of a pipeline, to monitor corrosion damage to the inside of the pipe walls.)12) Acoustic Emission (AE) (non-intrusive) (measurement of acoustic sound waves emitted during the growth of microscopic defects, such as stress corrosion cracks. The sensors act as microphones, positioned on structures, which detect sound waves generated from mechanical stresses generated during pressure or temperature changes.)B. Indirect techniques1) Corrosion Potential (non-intrusive) (The corrosion potential of the element to be monitored is measured relative to a reference electrode, which is characterized by a stable half-cell potential.)2) Hydrogen Monitoring (non-intrusive) (The generation of atomic hydrogen, as part of the cathodic half-cell reaction in acidic environments. Hydrogen monitoring probes are based on either of the following three principles: pressure increase, electrochemical current resulting from the oxidation of hydrogen under an applied potential, and current flow in an external circuit, based on a fuel cell principle.)3) Chemical Analyses (e.g., pH, conductivity, dissolved oxygen, metallic and other ion concentrations, water alkalinity, concentration of suspended solids, inhibitor concentrations and scaling indices)4) Corrosion sensors are typically electrochemical, and rely on changes in conductivity, resistivity, or redox potential to determine or predict corrosion. See, G. D. Davis, C. M. Dacres, and M. B. Shook, “Development of an electrochemistry-based corrosion sensor to monitor corrosion of boiler tubes, pipes, and painted structures”, DACCO SCI, INC. (www.daccosci.com/SPIE98.htm), expressly incorporated herein by reference.5) Certain “magnetic” corrosion sensors are known, for example a mechanical breakage sensor whose output is magnetically coupled through a vessel wall. See, Dennis G. Douglas, Christopher M. Smith, Phillip C. Ohl, Carey M. Haas (Vista Engineering Technologies, LLC), “CORROSION MONITORING OF PLUTONIUM OXIDE AND SNF”, WM'03—Topic No. 2.4, Technical Progress . . . Long Term Storage of SNF, www.vistaengr.com/library/CorrosionMonitoringofPlutoniumOxideandSNF.pdf.6) In addition, a number of sensors detect a current or magnetic field induced by a current with a superconducting quantum interference device (SQUID). See, e.g., Hitoshi Yashiro, Masahito Yoshizawa, Naoaki Kumagai, and Johann H. Hinken, “Effect of Spatial Distribution of Electronic and Ionic Currents on the Magnetic Field Induced by Galvanic Corrosion”, J. Electrochem. Soc., Volume 149, Issue 3, pp. B65-B69 (March 2002); Eimutis Juzeliunas, Meilute Samuleviciene, Aloyzas Sudavicius, and Johann H. Hinken, “A SQUID Study of Magnetic Fields Resulting from In Situ Corrosion Reactions”, Electrochem. Solid-State Lett., Volume 3, Issue 1, pp. 24-27 (January 2000); Weiss, Rick, “Taking corrosion's magnetic pulse. (research on monitoring magnetic fields associated with corrosion)”, Science News; Feb. 27, 1988.
Recent progresses in magnetic field sensing, and magnetic materials research lend new technologies in monitoring corrosion. One idea consists of detecting changes in magnetic properties due to corrosion of specially designed magnetic materials. Because the corrosion products of magnetic materials are generally nonmagnetic or with distinctly different measuring the changes in these properties by corrosion can be a sensitive tool.
The Giant Magneto Resistance (GMR) effect was discovered in 1988 and it is the phenomenon where the resistance of certain materials drops dramatically as a magnetic field is applied. It is described as Giant since the effect is much larger than in regular metals. It has generated strong interests in research community, as there are perspectives both in exploring new physics and technological applications as in magnetic recording and sensors. The effect is seen in spin-valve structures, where two magnetic layers are closely separated by a thin non-magnetic spacer of a few nanometer thickness. It is analogous to a light polarization experiment, where aligned polarizers allow light to pass through, but crossed polarizers do not. A magnetic layer allows electrons in only one spin state to pass through easily. If the second magnetic layer is aligned then that spin channel can easily pass through the structure, and the resistance is low. If the second magnetic layer is misaligned then neither spin channel can get through the structure easily and the electrical resistance is high. The GMR effect effectively measures the difference in the angle between the two magnetic orientations in the magnetic layers, with small angles (parallel) giving low resistance, and large angles (antiparallel) giving higher resistance. This angle depends on the applied magnetic field, and thus the change in the magnetoresistance can be used to measure magnetic fields. In GMR spin-valve materials ΔR/R values of more than 65% at the ambient temperature have been reported. Since the sensitivity of a sensor is proportional to ΔR/R, the advantages of these GMR materials for field sensing is obvious.
U.S. Pat. No. 6,599,401, Wang, et al. (having a common inventor with the present application), expressly incorporated herein by reference, provides an in-plane anisotropic tri-layered magnetic sandwich structure which demonstrates a large magnetoresistance effect. Fe/Co/Cu/Co magnetoresistive structures deposited on Si (100) substrates were found to demonstrate uniaxial magnetic anisotropy. Samples magnetized along an easy anisotropy axis showed extremely sharp magnetization, and corresponding magnetoresistance, switching at low fields and maximum giant magnetoresistance of 9.5% at 5K (5.5% at room temperature) for the samples with 5 nm of Fe, 5 nm of Co, 2.5 nm of Cu and 2 nm of Co.
A GMR effect sensor is proposed in Wang et al., U.S. Pat. No. 6,462,541, expressly incorporated herein by reference. Wang et al., suggest a uniform sense condition magnetic field sensor using differential magnetoresistance. A ferromagnetic thin-film based sensing arrangement having a plurality of magnetic field sensors on a substrate, each having an intermediate layer of a nonmagnetic material with two major surfaces on opposite sides thereof, with one of a pair of permeable films each formed of a magnetoresistive, anisotropic ferromagnetic material correspondingly positioned thereon, with first and second oriented sensors therein respectively having a selected and a reversing magnetization orientation structure provided with one of the pair of permeable films thereof, for orienting its magnetization in a selected direction absent an externally applied magnetic field in at least partly opposing directions. The magnetizations of those films rotates over a smaller angle in a selected externally applied magnetic field present thereat than does the magnetization orientation of the other permeable film in the pair. The first and second oriented sensors are electrically connected between a pair of terminating regions suited for electrical connection across a source of electrical energization. Alternatively, these magnetizations of the films can be oriented in the same direction but with the other film member of the pair provided adjacent a coupling layer that antiferromagnetically couples thereto a further ferromagnetic layer on an opposite side thereof of a lesser thickness for one sensor, and a greater thickness for the other. The intermediate layer material can be either a conductive material or a dielectric material. Such a sensing arrangement can be formed by providing a succession of material layers on a substrate having therein the intermediate layer and the pair of permeable films, with the one of the pair of permeable films being adjacent a succession of magnetization orientation layers having at least one coupling layer for antiferromagnetically coupling ferromagnetic layers on opposite sides thereof. Removal of some of the succession to provide an unequal number of coupling layers between locations for the two kinds of sensors, or unequal thicknesses of corresponding ferromagnetic layers corresponding to the coupling layer, is followed by providing a pinning layer at both kinds of locations. Removal of the succession at other locations results in providing the two kinds of sensors.
U.S. Pat. No. 6,600,637 (Wang et al.), expressly incorporated herein by reference, relates to a magnetoresistive sensor for a magnetic storage system having an edge barrier to prevent spin valve corrosion. During fabrication of a transducing head, Wang et al. report that the MR sensor is subjected to many processing steps. Current contacts and biasing layers are commonly deposited adjacent to the MR sensor after the MR sensor is shaped, but before the second half gap is deposited. The formation of the contacts and biasing layers, as well as the patterning of the MR sensor itself, subjects the MR sensor to a harsh environment that may result in corrosion of the MR sensor. This is particularly true of a multi-layered sensor such as a spin valve sensor.
Multi-layered sensors generally are formed of multiple materials, several of which very easily corrode. Since an MR sensor relies on the existence of each of its layers to operate properly, corrosion of any of its layers will result in the sensor having a reduced amplitude, a distorted signal output, decreased stability, and/or increased noise. They do not suggest that this corrosion may be intentionally exploited, nor that conditions other than those of manufacture may have an effect on the sensor.
Spin valves are a type of solid state magnetic field sensors. See, www.stoner.leeds.ac.uk/research/spinv.htm. A spin valve is, in general, a sample consisting essentially of a GMR trilayer. One layer is magnetically soft—meaning its magnetization is very sensitive to small fields. The other is made magnetically ‘hard’ by various schemes—meaning it is insensitive to fields of moderate strength. As the soft ‘free’ layer changes its magnetic state, due to applied fields, the resistance of the whole structure will vary. The central part of the sample consists of two magnetic layers (e.g., permalloy with a thin covering of Co), separated by a Cu spacer layer. One magnetic layer is pinned or exchange biased by an antiferromagnetic material, e.g., FeMn and IrMn.
Spin valves are dedicated GMR systems, in which the electrical resistance is high or low, depending on the direction rather than the strength of the magnetic field. Contrary to water valves they are not fully open or closed, the change in resistance is typical in the range of 5% to 10%. A spin valve is typically made of only two ferromagnetic layers spaced by a layer of nonmagnetic metal. Contrary to a GMR multilayer, the two ferromagnetic layers are magnetically coupled. As a further difference the magnetization of one of the ferromagnetic layers is spatially fixed (“pinned”) by an antiferromagnetic bottom layer. Thus it is called the “pinned layer”, the other is called the “free layer”, because it should easily follow the external magnetic field.
The resistance in the plane of a ferromagnetic thin-film is isotropic with respect to the GMR effect rather than depending on the direction of a sensing current therethrough as for the anisotropic magnetoresistive effect. The GMR effect has a magnetization dependent component of resistance that varies as the cosine of the angle between magnetizations in the two ferromagnetic thin-films on either side of an intermediate layer. In the GMR effect, the electrical resistance through the “sandwich” or superlattice is lower if the magnetizations in the two separated ferromagnetic thin-films are parallel than it is if these magnetizations are antiparallel, i.e. directed in opposing directions. Further, the anisotropic magnetoresistive effect, also present in GMR structures in very thin-films, is considerably reduced from the bulk values therefor in thicker films due to surface scattering, whereas very thin-films are a fundamental requirement to obtain a significant GMR effect. The GMR effect can be increased by adding further alternate intermediate and ferromagnetic thin-film layers to extend the “sandwich” or superlattice structure. The GMR effect is sometimes called the “spin valve effect” in view of the explanation that a larger fraction of conduction electrons are allowed to move more freely from one ferromagnetic thin-film layer to another if the magnetizations in these layers are parallel than if they are antiparallel with the result that the magnetization states of the layers act as sort of a valve.
In magnetic multilayers, these magnetization configurations often come about because of magnetic exchange coupling between the ferromagnetic thin-films separated by the intermediate layers, these intermediate layers typically formed from a nonferromagnetic transition metal. The effect of the exchange coupling between the ferromagnetic thin-film layers is determined to a substantial degree by a function of the thickness of the intermediate layer, which oscillates as a function of the separation thickness between ferromagnetic coupling and antiferromagnetic coupling. In spin valves, such exchange coupling is not as obvious, although Wang reported exchange coupling does affect the soft magnetic layer switching with applied magnetic field in pseudo spin valves.
The ferromagnetic thin-film layers may be formed with alternating high and low coercivity materials so that the magnetization of the low coercivity material layers can be reversed without reversing the magnetizations of the others. An alternative arrangement is to provide “soft” ferromagnetic thin-films and couple antiferromagnetically every other one of them with an adjacent magnetically hard layer (forming a anti-ferromagnetic thin-film double layer) so that the anti-ferromagnetic double layer will be relatively unaffected by externally applied magnetic fields even though the magnetizations of the other ferromagnetic thin-film layers will be subject to being controlled by such an external field. A multilayer structure may be provided that is etched into strips such that demagnetizing effects and currents can be used to orient the magnetizations antiparallel, and so that externally applied magnetic fields can orient the magnetizations parallel. Thus, parallel and antiparallel magnetizations can be established in the ferromagnetic thin-films of the structure as desired in a particular use. Such a structure must be fabricated so that any ferromagnetic or antiferromagnetic coupling between separated ferromagnetic films is not too strong so as to prevent such establishments of film magnetizations using practical interconnection arrangements.
A magnetic field sensor suited for fabrication with dimensions of a few microns or less can be fabricated that provides a suitable response to the presence of very small external magnetic fields and low power dissipation by substituting an electrical insulator for a conductor in the nonmagnetic layer. This sensor can be fabricated using ferromagnetic thin-film materials of similar or different kinds in each of the outer magnetic films provided in a “sandwich” structure on either side of an intermediate nonmagnetic layer which ferromagnetic films may be composite films, but this insulating intermediate nonmagnetic layer permits electrical current to effectively pass therethrough based primarily on a quantum electrodynamic effect “tunneling” current.
This “tunneling” current has a magnitude dependence on the angle between the magnetization vectors in each of the ferromagnetic layers on either side of the intermediate layer due to the transmission barrier provided by this intermediate layer depending on the degree of matching of the spin polarizations of the electrons tunneling therethrough with the spin polarizations of the conduction electrons in the ferromagnetic layers, the latter being set by the layer magnetization directions to provide a “magnetic valve effect”. Such an effect results in an effective resistance, or conductance, characterizing this intermediate layer with respect to the “tunneling” current therethrough. The maximum fractional change in effective resistance is a function of the magnetic polarization of the conduction electrons given by (ΔR/R)=2P1 P2/(1+P1P2), where P1 and P2 are the conduction electron spin polarizations of the two ferromagnetic layers. These polarizations appear dependent on the ratio of spin up to spin down electrons in the 3D shell of the transition elements used in the ferromagnetic thin-films, i.e. the spin polarization P of the conduction electrons. The fraction f of 3D electrons which are spin up have typical values of 0.75 for iron, 0.64 for cobalt and 0.56 for nickel. Conduction electrons in metals are normally S shell electrons which theoretically would be equally divided between spin up and spin down electrons. However, because of band splitting the conduction electrons in the magnetic layers are assumed to have a fraction of spin up electrons like that of the electrons in the 3D shell. The spin polarization is then determined from P=2f−1.
In addition, shape anisotropy is often used in such a sensor to provide different coercivities in the two ferromagnetic layers, and by forming one of the ferromagnetic layers to be thicker than the other. Such devices may be provided on a surface of a monolithic integrated circuit to thereby allow providing convenient electrical connections between each such sensor device and the operating circuitry therefor.
A “sandwich” structure for such a sensor, based on having an intermediate thin layer of a nonmagnetic, dielectric or otherwise, separating material with two major surfaces on each of which a anisotropic ferromagnetic thin-film is positioned, exhibits the “magnetic valve effect” if the materials for the ferromagnetic thin-films and the intermediate layers are properly selected and have sufficiently small thicknesses. The resulting “magnetic valve effect” can yield a response which can be several times in magnitude greater than that due to the “giant magnetoresistive effect” in a similar sized sensor structure.
The current-voltage characteristics of such “sandwich” structure sensors will exhibit a relatively linear change in the quantum electrodynamic effect “tunneling” current therethrough from one ferromagnetic layer through the barrier to the other with respect to the voltage provided across the sensor, i.e. across the barrier layer between these ferromagnetic layers, for relatively lower value voltages, but the current magnitude increases more than linearly for higher values of voltage across the sensor. As the voltage across the sensor increases, the fractional change in the “tunneling” current through the sensor, for the ferromagnetic layers having magnetizations changing from parallel to one another to antiparallel, decreases to being only half as great with several hundred millivolts across the sensor as occurs in the situation with a hundred or less millivolts across the sensor so that this fractional change with sensor voltage will range from a few percent to 20% or more. The fractional change in the resistance of the sensor for the ferromagnetic layers having magnetizations changing from parallel to one another to antiparallel increases to about one and one-half the room temperature values when the sensor is cooled to 77° K, but the “tunneling” current through the sensor increases by only about 10% to 20% indicating that the effective resistivity of the sensor is relatively insensitive to temperature (around 500 to 1000 ppm/° C.).
The effective resistivity of such a sensor is set by the amount of “tunneling” current through the cell permitted by barrier layer for the voltage across the sensor. The high sensitivity of the “tunneling” current to the thickness of the barrier layer leads to a wide range of sensor resistivities which have been observed to be from 60.0Ω-μm2 to 10,000 MΩ-μm2. On the other hand, the barrier layer appears to permit relatively weak magnetic coupling between the ferromagnetic layers thereacross with the coupling fields typically being only a few Oe.
The barrier material for such sensing devices has typically been aluminum oxide, Al2O3 and other such oxides, but other dielectric materials have been used. A typical construction therefore has had two long rectangular ferromagnetic thin-film strips with the barrier layer therebetween such that the long axis of the bottom strip, supported directly on an electrically insulating substrate, at some angle with respect to that of the upper strip supported thereon through the barrier layer. This arrangement leaves the crossover area where these ferromagnetic strips overlap having the shape of a parallelogram defining the portion of the barrier layer through which there is effective current tunneling between the strips.
These devices are fabricated by depositing upon the insulating substrate a narrow stripe of the bottom ferromagnetic film typically using a separate, removable mask. A layer of dielectric material is then formed over this bottom film, and then a second narrow stripe ferromagnetic film is deposited through a mask such that the long direction axis of the second stripe is, typically, perpendicular to that of the first. The region of tunneling between the two stripes is then typically shaped as square or rectangle where the two stripes overlap. The shape of the interposed dielectric barrier is inconsequential so long as it is sufficiently large to completely separate the two ferromagnetic thin-film metal stripes. The ferromagnetic layers in these structures are typically simple single films of Fe, Co, NiFe or other common ferromagnetic alloys.
Generally, fabricating a very small overlap area in such sensors using masking techniques is difficult to accomplish because of deposition material spatial distribution variances which can lead to electrical short circuits between the strips. As a result, overlap area, or tunnel junction, dimensions are often of many millimeters in length and relatively thick barrier layers are needed.
The operating current for such sensors is typically supplied through a pair of current leads with one such lead connected to an end of the upper strip and the other lead connected to an end of the lower strip. The effective electrical resistance of the sensor is determined from measuring the voltage across the tunnel junction at two voltage leads each connected to one of the remaining two ends of these strips. Then, by providing a current of a known fixed value through the current leads and measuring the corresponding tunnel junction voltage on the voltage leads, the effective resistance can be simply calculated by dividing the measured voltage value by the chosen fixed current value.
Because, as indicated above, the conduction of current across the barrier of such a sensor is due to a quantum electrodynamic tunneling effect, the conduction turns out to be highly dependent on the thickness of the barrier. An increase of 2 Å in the barrier thickness can lead to an increase the junction resistance by a factor of 10. The measured resistances of tunnel junctions fabricated from the same starting material are inversely proportional to the areas of those junctions. Typical tunneling resistivities (ρT, calculated by multiplying the resistance by the tunnel junction area) range from 10−2 to 10−3 MΩ-μm2. These resistivities correspond to Al2O3 thickness of about 12 to 30 Å, respectively. Due to the sharp dependence of tunnel resistivity on the barrier thickness, ρT can easily vary across a single wafer by a factor of two.
As indicated above, the measured resistance of the tunnel junction in such a sensor is a function of the relative orientation of the magnetizations of the two ferromagnetic thin-film metal strips. The portion of the tunnel junction resistance that is subject to change as a result of that junction experiencing changes in external magnetic fields to which it is exposed is termed junction magnetoresistance (often written JMR, and defined as ΔR/Rmin but is equivalently ΔR/Rmin for voltage measurements with a fixed current with either being expressed as a percentage). The sensors described above demonstrated that the JMR therefore can be quite large at room temperature (>25%).
However, such sensors cannot be conveniently incorporated into integrated circuits because the sputter-mask mode of fabrication is not compatible with modern semiconductor fabrication. In addition, the magnetic response of these sensors is not optimized for applications. In particular, they exhibit considerable hysteresis, nonlinearity and other nonideal aspects in their JMR response, including small signal output and low areal density, as have the tunnel junction field sensor structures of subsequent designs.
A better magnetic field sensor can be made using modem semiconductor fabrication techniques having a junction structure in a sensor cell based on a nonmagnetic intermediate separating material with two major surfaces on one of which is a base anisotropic ferromagnetic thin-film which is also on a base electrode, and on the other of which there is at least one of a plurality of separate anisotropic ferromagnetic thin-films but of differing effective coercivities. The nonmagnetic intermediate separating material can be either a conductive material leading to a GMR device or an insulator leading to a spin dependent tunneling device. Similar structures have a separate film in each that can be interconnected to one another with the interconnections extending at least in part substantially parallel to the widths of the separated films. The base electrode and the separated films can have lengths with gradually narrowing widths toward each end which narrow to zero at the ends. The intermediate material supported on a base electrode can be common to all the separated films thereon.
Often more than one such magnetic field sensor is used in a sensing configuration to provide a larger output signal and, in many instances, to provide some sensor noise cancellation. These goals are many times pursued through use of a bridge circuit in which such GMR effect structures or spin dependent tunneling structures are provided as circuit resistors connected in two parallel branches between two power supply nodes with each such branch having two such resistors in series with one another. A single polarity voltage source is typically connected between the two power supply nodes with in many instances the negative side of the source being grounded. A signal sensing differential amplifier with a pair of inputs is typically electrically connected between the two bridge circuit output nodes, i.e. the internal nodes of each of the two branches which for each is the node between the two resistors connected in series therein.
To have such a bridge circuit operate properly, adjacent ones of the magnetoresistors in the circuit must vary in resistance differently under an applied magnetic field if a signal output is to result. If they each have the same resistance variation, there will be a zero value signal developed between the bridge circuit output nodes, i.e. between the sensing amplifier inputs. Since an externally applied magnetic field to be sensed will be approximately the same for each of the closely spaced resistors in the bridge circuit, design measures are necessary to assure the needed resistive differences nevertheless occur between the adjacent circuit structures or resistors. One such measure previously used has been to place two of these magnetoresistors on opposite sides of the bridge circuit each connected to different power supply terminals under a magnetic shield leaving only the other two such resistors exposed to the effects of externally applied magnetic fields. Such an arrangement, however, results in a smaller output signal for an applied eternal field than would otherwise be possibly available since two resistors are not being used to sense that field. Furthermore, provision of such shields adds risk to the fabrication process since they must be relatively large structures formed after most other steps are completed. Thus, there is a desire to obtain the needed resistive differences between the adjacent circuit structures or magnetoresistors while obtaining the full possible output signal without having to fabricate shielding structures.